In Escherichia coli, replication of the terminal segment of the chromosome initiates the subsequent septum formation and cell division. An approach to the study of how septum formation is regulated is by the use of conditional cell division mutants. Temperature-sensitive mutants defective in the ftsA and sep genes are defective specifically in septum formation. The sep product is required continuously during septation; the ftsA product is necessary to initiate septum formation. Sep and ftsA are located near leu and in a cluster with other genes involed in membrane-wall biosynthesis and function. From an unusual lysogen with lambda integrated in lambda transducing phages which carry sep plus and ftsA plus have been isolated. The phages will be used to provide a genetic and physical map of the sep-ftsA region by transduction and heteroduplex analysis. The possibility that some or all the genes of this region are coordinately expressed will be investigated by identifying and sizing the mRNA's specific for the sep-ftsA region. The functions of sep and ftsA in division will be determined. Plaque-forming lambda sep plus and lambda ftsA plus phages and nonsense mutants isolated from them (lambda sep minus am and lambda ftsA minus am) will be used to identify the products of sep and ftsA by labelling proteins synthesized from these phages. The possibility that septum formation is regulated by cyclic expression of sep and/or ftsA will be investigated by measuring the amount of each mRNA present during stages of the cell cycle. Radioimmune assays for sep and ftsA proteins will determine how their levels fluctuate during the cell cycle. The time during the cycle when the sep and ftsA proteins must function will be determined by temperature shifts of synchronized cultures. Another conditional septum formation mutant is the lon mutant. A molecular approach to defining the lon function is provided by the fact that lambda repressor synthesis is inhibited after infection of lon hosts. How the lon defect affects lambda repressor synthesis will be determined.